In the existing commercial aviation communication system, the capabilities of aircraft have been stifled by the lack of ability to send digital data between aircraft and/or ground stations. In fact, there are currently no means for general purpose digital data communication in the VHF (very high frequency) band dedicated to this system and covering the frequency spectrum (i.e., bandwidth) between 108 and 136 MHz. This is primarily due to the lack of available spectrum in the VHF band.
One of the major benefits of having digital data communication between aircraft and ground stations is the increase in aviation safety. It is also highly desirable to have digital data communication because of the vast array of new services that can be offered to the commercial aviation industry. Table 1 shows the services currently available without digital data communication and the services that would be available with digital data communication.
TABLE 1Service (i.e., Communication Type)WithoutWithVoiceXXMessage QueuingXMessage PrioritizationXMessage AuthenticationXMessage AddressingXMessage EncryptionXPassive Hand-oversXATIS (Automated Terminal Information Service)XXBasic Weather RadarXCollision AvoidanceXPosition TriangulationXAirborne Weather StationXXGround Radar Back-UpXAircraft Position LocationXDME FunctionsX
Completely switching to a new digital data communication system poses many problems. For example, it is not practical to force aircraft owners and small airports to sacrifice their investment in current avionics and purchase replacement avionics overnight. Moreover, any aircraft in flight or in operation on the ground at a landing facility that does not transition to the new system posses a serious threat to itself and other aircraft. The potential for mid-air collisions and collisions on the ground is unacceptable in this case.
Therefore, for practical and safety reasons, a migration path to the new system is needed which does not disrupt operation of the existing commercial aviation communication system. The simplest way to provide such a migration path is through coexisting communication systems. In this way, a new digital data communication systems can be added to the existing avionics communication system and users can naturally migrate to the new system as their budgets allow.
One solution for migrating to a digital data communication system is to add digital channels in the VHF frequency band, as proposed in “VHF Air-Ground Communication”, Document No. RTCA/SC-172, Washington, D.C., 1994. In this solution, existing AM communication and/or navigation channels in the 108 to 118 MHz frequency band would be de-allocated and then re-allocated for digital channels. These digital channels could be used for both vocal and data digital data communication.
There are several draw backs to this solution. First, presently allocated channels must be de-allocated. This disrupts existing communication and makes the task of updating and switching the industry very costly. In addition, anyone who is unaware of the change will not be able to communicate properly. This presents the opportunity for disaster. Second, existing equipment will become obsolete. This will cause costly avionics upgrades to be made to all aircraft and ground units. This poses a problem to the airlines and especially to the small airline operator and general aviation industry. Scraping the existing equipment can be avoided if existing channels are relocated. But, as just mentioned, this is undesirable.